It is often desirable to join two or more components together by welding. One known welding technique comprises diffusion bonding. Diffusion bonding is a solid state welding technique, in which heat and high pressure is applied to the components to be bonded in a direction normal to the bond interface at high temperatures, such that atoms of respective bonding surfaces of the articles intermingle over time to form a bonded article.
In some cases, an interlayer is provided between the first and second articles in order to provide improved alignment between the first and second articles. Such a method is described in US 2011035924, in which a powder metal interlayer is interposed between the respective bonding surfaces. In one embodiment, an interlayer in the form of a preform metal powder ring is interposed between the bonding surfaces and subsequently heated and pressed by the other article to form a consolidated interlayer bonded to the respective bonding surfaces of the articles. The powdered metal interlayer may be partially consolidated and formed via conventional rolling or cold compression, before being placed between the articles and subjected to heat and pressure to diffusion bond the articles together.
However, in the case of US2011035924 it is limited in its range of application having a bond of limited strength, and is constrained by its requirement to have a symmetrical primitive geometry.
It has been found by the inventors that, even where the first and second articles are polished to a low surface roughness, some protrusions in the surfaces of the first and second articles remain. Where electrical resistance heating is used to heat the articles and interlayer to the required temperature to initiate diffusion bonding, electrical arcing between the protrusions can lead to localised heat spikes in the first and second articles. It is thought that these heats spikes cause localised melting during the bonding process, which may result in differentiation of the alloy, and so a weakened bond in those areas.
The present invention describes a bonding method which seeks to overcome some or all of the above problems.